Electron beam exposure system

ABSTRACT

The invention relates to an electron beam exposure apparatus for transferring a pattern onto the surface of a target, comprising:
         a beamlet generator for generating a plurality of electron beamlets;   a modulation array for receiving said plurality of electron beamlets, comprising a plurality of modulators for modulating the intensity of an electron beamlet;   a controller, connected to the modulation array for individually controlling the modulators,   an adjustor, operationally connected to each modulator, for individually adjusting the control signal of each modulator;   a focusing electron optical system comprising an array of electrostatic lenses wherein each lens focuses a corresponding individual beamlet, which is transmitted by said modulation array, to a cross section smaller than 300 nm, and   a target holder for holding a target with its exposure surface onto which the pattern is to be transferred in the first focal plane of the focusing electron optical system.

Notice: More than one reissue application has been filed for the reissueof U.S. Pat. No. 7,091,504. The reissue applications are applicationSer. No. 12/189,817 (the present application), Ser. Nos. 13/343,036, and13/343,038, all of which are divisional reissues of U.S. Pat. No.7,091,504.

The present patent application is a Divisional of application Ser. No.10/699,246 filed Oct. 30, 2003, now U.S. Pat. No. 6,897,458 which is aNon-Provisional of Provisional Application No. 60/422,758 filed Oct. 30,2002.

BACKGROUND

Several kinds of electron beam exposure systems are known in the art.Most of these systems are provided to transfer very precise patternsonto an exposure surface of a substrate. Since lithography features arepushed to become smaller and smaller following Moore's law, the highresolution of electron beams could be used to continue the drive to evensmaller features than today.

A conventional electron beam exposure apparatus has a throughput ofabout 1/100 wafer/hr. However, for lithography purposes a commerciallyacceptable throughput of at least a few wafers/hr is necessary. Severalideas to increase the throughput of an electron beam exposure apparatushave been proposed.

U.S. Pat. No. A1-5,760,410 and U.S. Pat. No. A1-6,313,476, for instance,disclose a lithography system using an electron beam having a crosssection, which is modified during the transferring of a pattern to anexposure surface of a target. The specific cross section or shape of thebeam is established during operation by moving the emitted beam insidean aperture by using electrostatic deflection. The selected aperturepartially blanks and thereby shapes the electron beam. The targetexposure surface moves under the beam to refresh the surface. In thisway a pattern is written. The throughput of this system is stilllimited.

In US A1-20010028042, US-A1-20010028043 and US-A1-20010028044 anelectron beam lithography system is disclosed using a plurality ofelectron beams by using a plurality of continuous wave (CW) emitters togenerate a plurality of electron beamlets. Each beamlet is thenindividually shaped and blanked to create a pattern on the underlyingsubstrate. As all these emitters have slightly different emissioncharacteristics, homogeneity of the beamlets is a problem. This wascorrected by levelling every individual beam current to a referencecurrent. Correction values for the mismatch are extremely difficult tocalculate and it takes a significant amount of time, which reduces thethroughput of the system.

In Journal of Vacuum Science and Technology B18 (6) pages 3061–3066, asystem is disclosed which uses one LaB₆-source for generating oneelectron beam, which is subsequently, expands, collimated and split intoa plurality of beamlets. The target exposure surface is mechanicallymoved relatively to the plurality of beamlets in a first direction, thebeamlets are switched on and off using blanking electrostatic deflectorsand at the same time scanning deflectors sweep the beamlets which havepassed the blanker array over the target exposure surface in a directionperpendicular to the first direction, thus each time creating an image.In this known system, electrostatic and/or magnetic lenses are used toreduce the image before it is projected on the target exposure surface.In the demagnification process at least one complete intermediate imageis created, smaller than the one before. When the entire image has thedesired dimensions, it is projected on the target exposure surface. Amajor disadvantage of this approach is that the plurality of electronbeamlets together has to pass through at least one complete crossover.In this crossover, Coulomb interactions between electron in differentbeamlets will disturb the image, thus reducing the resolution. Moreover,due to the strong demagnification of the image, the area that is exposedat one time is rather small, so a lot of wafer scans are needed toexpose a die: 16 scans are needed to expose one die, requiring a veryhigh stage speed for reaching a commercially acceptable throughput.

In GB-A1-2.340.991, a multibeam particle lithography system is disclosedhaving an illumination system, which produces a plurality of ionsub-beams. The illumination systems use either a single ion source withaperture plates for splitting a beam in sub-beams, or a plurality ofsources. In the system using a single ion source, the aperture plate isprojected (demagnified) on a substrate using a multibeam optical system.The system furthermore uses a deflection unit of electrostatic multipolesystems, positioned after the multibeam optical system, for correctingindividual imaging aberrations of a sub-beam and positioning thesub-beam during writing. The publication does not disclose how eachsub-beam is modulated. Furthermore, controlling individual sub-beams isa problem, and maintaining inter-sub-beam uniformity.

In Jpn. J. Appl. Phys. Vol. 34 (1995) 6689–6695, a multi-electron beam(‘probes’) lithography system is disclosed having a specific ZrO/W-TFEthermal emission source with an emitter tip immersed in a magneticfield. A disadvantage of such a source is its limited output.Furthermore, this source needs a crossover. The mutual homogeneity ofthe ‘probes’ is not further discussed. Furthermore, the intensity of thesource is a problem.

The article furthermore in a general way mentions a writing strategy inwhich a stage is moved in one direction, and deflectors move the‘probes’ concurrently through the same distance perpendicular to thedirection of the stage movement. A further problem, not recognised inthis publication, is correction of deviation of electron beamlets fromtheir intended positions.

SUMMARY OF THE INVENTION

It is an objective of the current invention to improve the performanceof known electron beam exposure apparatus.

Another objective is to improve the resolution of known electron beamexposure apparatus.

Yet another objective of the current invention is to improve throughputof known electron beam exposure apparatus.

Yet another objective of the current invention is to overcome theproblems related to Coulomb interactions and the demagnification methodsin the prior art.

Another objective of the current invention is to simplify controllinguniformity of beamlets, especially during writing.

The invention relates to an electron beam exposure apparatus fortransferring a pattern onto the surface of a target, comprising:

-   -   beamlet generator for generating a plurality of electron        beamlets;    -   a modulation array for receiving said plurality of electron        beamlets, comprising a plurality of modulators for modulating        the intensity of an electron beamlet;    -   a controller, operationally connected to the modulation array        for individually controlling the modulators using control        signals;    -   an adjustor, operationally connected to each modulator, for        individually adjusting the control signal of each modulator;    -   a focusing electron optical system comprising an array of        electrostatic lenses wherein each lens focuses a corresponding        individual beamlet, which is transmitted by said modulation        array, to a cross section smaller than 300 nm, and    -   a target holder for holding a target with its exposure surface        onto which the pattern is to be transferred in the first focal        plane of the focusing electron optical system.

In this apparatus, electron crossover could be avoided, as it does notdemagnify a complete (part of) an image. In this way, resolution andwriting speed increases. Furthermore, it avoids the needs to control thecurrent in each individual beamlet. The apparatus is less complex as theposition correction and modulation are integrated.

In an embodiment of an electron beam exposure apparatus according to thepresent invention, said modulation array comprises:

-   -   a beamlet blanker array comprising a plurality of beamlet        blankers for the deflection of a passing electron beamlet,    -   a beamlet stop array, having a plurality of apertures aligned        with said beamlet blankers of said beamlet blanker array.

In this way, it is possible to avoid crossover of electron beamlets inone single focal point, and make high-speed modulation possible. In anembodiment, substantially every beamlet blanker is aligned with anelectron beamlet, in order to make it possible to individually modulateevery beamlet. Furthermore, the beamlet stop array comprises at leastone plane of apertures, substantially every aperture being aligned withone beamlet, preferably with an aperture centred with respect to abeamlet. In this way, a beamlet passes an aperture when an electronbeamlet is not deflected, and a beamlet is blocked or stopped when thebeamlet is deflected. In an embodiment of this modulation array, thecontroller is operationally connected to said beamlet blankers.

In an embodiment, the electron beam exposure apparatus is furthermoreprovided with measuring means for measuring the actual position of atleast one of said beamlets, and the controller is provided with memorymeans for storing said actual position and a desired position, acomparator for comparing the desired position and the actual position ofsaid beamlets, and wherein the adjustor is operationally connected tothe controller for receiving instructions for adjusting the controlsignals issued to the modulators to compensate for the measureddifference between said desired position and said actual position ofsaid electron beamlets. In this way, by adjusting control signals,positioning of the beamlets can be corrected in an easy way. Measurementof the actual positions can for instance be done as described in U.S.Pat. No. A1-5,929,454.

In an embodiment, the controller is operationally connected to thebeamlet blankers, in an embodiment via the adjustor.

In an embodiment, the adjustor is operationally connected to thecontroller for receiving instructions indicating the amount of theadjustments. The amount of the adjustments can be determined based aresulting value of the above-mentioned comparator.

In a further embodiment, the adjustor is adapted for individuallyadjusting timing of each control signal. In this very easy way,correction can be accomplished.

In an embodiment of the electron beam exposure apparatus according tothe present invention, the beamlet generating means comprise:

-   -   a source for emitting at least one electron beam,    -   at least one beamsplitter for splitting said at least one        emitted electron beam into said plurality of electron beamlets

In this way, a uniform intensity distribution among the beamlets iseasily achieved if the source emits uniformly in all relevantdirections. In an embodiment, the electron beam exposure apparatusfurther comprising a second electrostatic lens array located betweensaid beam splitting means and said beamlet blanker array to focus saidplurality of electron beamlets. In this embodiment, substantially everyelectrostatic lens is aligned and focuses one electron beamlet. In afurther embodiment thereof, the beamlet blanker array is located in thefocal plane of said second electrostatic lens array.

In an embodiment of the electron beam exposure apparatus of the currentinvention with beamsplitter, the beamsplitter comprise a spatial filter,preferably an aperture array. In this way, one source with one beam, or,when source intensity is insufficient or intensity fluctuates across thebeam, several sources, are easily split into a plurality of beamlets.

When source intensities are high, the splitting means can comprise anumber of aperture arrays in a serial order along the path of theelectron beam or plurality of beamlets, the aperture arrays havingmutually aligned apertures, each next aperture array along the path fromthe source to the target having apertures that are smaller than theapertures of the previous aperture array. This reduces heat load.

In an embodiment of the aperture array, the apertures of each aperturearray are arranged in a hexagonal structure, which makes it possible toobtain close integration.

In a further embodiment of the electron beam exposure apparatuscomprising splitting means comprising an aperture array, each apertureof the aperture array has an area inversely proportional to the currentdensity based on the beamlet that is transmitted through that sameaperture.

In a further embodiment of the electron beam exposure apparatuscomprising a beamsplitter, the beamsplitter comprises an aperture array,wherein the aperture sizes in the aperture array are adapted to create adiscrete set of predetermined beamlet currents.

These embodiments improve the uniformity of the electron beamlets.

In yet a further embodiment of the electron beam exposure apparatuscomprising the beamsplitter, the beamsplitter comprises an electrostaticquadrupole lens array.

In an embodiment, the electron beam exposure apparatus according to thepresent invention comprises a thermionic source. In an embodiment, thethermionic source is adapted for being operated in the space chargelimited regime. It was found that space charge has a homogenisingeffect, which is favourable in this specific application. Furthermore,in certain settings, the space charge may have a negative lens effect.

In a further embodiment with the thermionic source, the thermionicelectron source has a spherical cathode surface. In an embodiment, thethermionic source comprises at least one extractor electrode. In anotherembodiment, the extractor electrode is a planar extractor electrode. Inan embodiment thereof, the extractor is located after the space chargeregion and provided with a positive voltage for inducing a negative lenseffect. These voltages can be set at a predefined value for creating anegative lens effect for the emitted electron beam.

In an alternative embodiment, the extractor electrode has a sphericalsurface with through holes. All these embodiments serve to create anegative lens influence on the electron beam, thus avoiding a crossoverin the electron beam.

In another embodiment of the electron beam exposure apparatus of thecurrent invention, the apparatus further comprises an illuminationsystem that transforms the electron beam, emitted by said source, into acollimated electron beam before it reaches said splitting means.

In yet another embodiment of the electron beam exposure apparatus saidbeamlet generator comprises an array of sources of which each source isresponsible for the generation of an electron beamlet. In a furtherembodiment thereof, the electron beam exposure apparatus furthercomprising a second electrostatic lens array located between said arrayof sources and said beamlet blanker array to focus said plurality ofelectron beamlets.

In an embodiment of the electron beam exposure apparatus with beamletblanking means, said beamlet blanker comprise electrostatic deflectors.

In yet another embodiment of the electron beam exposure apparatusaccording to the invention, it further comprising scanning deflectionmeans provided between the modulation array and the focusing electronoptical system for deflecting the electron beamlets to scan said targetexposure surface. In an embodiment thereof, the scanning deflectionmeans comprises electrostatic scan deflectors. In a further embodimentthereof, the electron beam exposure apparatus is further provided withactuating means for moving said electrostatic scan deflectors and saidmeans for holding the target relatively to each other in the plane ofthe surface onto which the pattern is to be transferred in a directionthat differs from the direction of the deflection performed by saidelectrostatic scan deflectors.

In an embodiment, the adjustor or a time shifter are adapted forshifting a timing base of the scanning deflection means and theactuators with respect to each other. In an embodiment thereof, thecontrol signals of the modulators have a timing base and the actuatorsof the target holder have a second timing base, and there timing basescan be shifted with respect to one another. This can for instance beused to have a critical component, which has to be written on the targetsurface and which would lay between two beamlets, written using only onebeamlet.

In a further embodiment thereof, the electron beam exposure apparatusfurthermore comprises an additional aperture plate between themodulation array and the focussing electron optical system, theadditional aperture plate having one surface directed to andsubstantially parallel to the exposure surface of the target, whereinsaid electrostatic scan deflectors are conducting strips deposited onthe side of the additional aperture plate facing the exposure surface ofthe target located between said blanker array and the electrostatic lensarray of the focusing electron optical system. In another embodimentthereof, the electrostatic scan deflectors are conducting stripsdeposited at the target exposure surface side of any of the lens platespresent in the focusing electron optical system. In an embodimentthereof, the conducting strips alternatively have a positive or negativepotential.

In an embodiment of the electron beam exposure apparatus with theblanking electrostatic deflectors, these deflectors deflect the electronbeamlets in such a way that a predetermined section of the beamlet isstopped by the beamlet stop array.

In a further embodiment of the electron beam exposure apparatusaccording to the present invention, it further comprises apost-reduction acceleration stage, located between the electrostaticlens array of the focusing electron optical system and said protectivemeans, for accelerating the electrons in the plurality of transmittedelectron beamlets.

In an embodiment of the controller, it is furthermore provided withcorrection means to compensate for the incorrect positioning of theelectron beamlets on the target exposure surface by

-   -   comparing the theoretical position and the actual position of        said beamlets    -   adjusting the control signals to compensate for the measured        difference between said theoretical position and said actual        position of said electron beamlets

In an embodiment of the electron beam exposure apparatus according tothe present invention, it further comprising protective means to preventparticles released by impinging electrons to reach any one of theaperture arrays, lens arrays or blanker arrays, preferably locatedbetween the electrostatic lens array of the focusing electron opticalsystem and the exposure surface of a target, preferably comprising anaperture array wherein the apertures have a size smaller than 20 μm.

In an embodiment of the electron beam exposure apparatus according tothe present invention, all lens arrays, aperture arrays and blankerarrays are connected to a power supply, which, when gas is admitted intothe system, creates a plasma that cleans the plates and removes allcontaminants.

In a further embodiment, the electron beam exposure apparatus accordingto the present invention, the system is operated at an elevatedtemperature of about 200–600° C. to keep the apparatus clean.

The invention further relates to an electron beam exposure apparatus fortransferring a pattern onto the surface of a target, comprising:

-   -   a beamlet generator for generating a plurality of electron        beamlets;    -   a modulation array for receiving said plurality of electron        beamlets, comprising a plurality of modulators for modulating        the intensity of an electron beamlet;    -   a controller, operationally connected to the modulation array,        for individually controlling the modulators using control        signals;    -   a focusing electron optical system comprising an array of        electrostatic lenses wherein each lens focuses a corresponding        individual beamlet, which is transmitted by said modulation        array, to a cross section smaller than 300 nm, and    -   a target holder for holding a target with its exposure surface        onto which the pattern is to be transferred in the first focal        plane of the focusing electron optical system,        wherein said beamlet generator comprises at least one thermionic        source, said source comprising at least one extractor electrode        adapted for being operated in a space charge limited region,        said source adapted for generating an electron beam, and said        beamlet generator furthermore provided with a beamsplitter for        splitting said electron beam up into a plurality of electron        beamlets.

Using such a specific beamlet generator makes it possible to provideuniform beamlets with a sufficient current to provide a high throughput.

In an embodiment thereof, said extractor electrode is located after saidspace charge region and is provided with a positive voltage for inducinga negative lens effect to said electron beam.

The invention furthermore pertains to an electron beam generator forgenerating a plurality of electron beamlets, wherein said beamletgenerator comprises at least one thermionic source, said sourcecomprising at least one extractor electrode adapted for being operatedin a space charge limited region, said source adapted for generating anelectron beam, and said beamlet generator furthermore provided with abeamsplitter for splitting said electron beam up into a plurality ofelectron beamlets.

The invention furthermore pertains to an electron beam exposureapparatus for transferring a pattern onto the surface of a target,comprising a beamlet generator for generating a plurality of electronbeamlets, a plurality of modulators for modulating each electronbeamlet, and a controller for providing each modulator with a controlsignal, said control signal having a timing base, wherein the controlleris adapted for individually adjusting the timing base of a controlsignal with respect to the other control signals.

In this apparatus, the problem of positioning and modulating is solvedin a very simple and elegant way, reducing the number of components andproviding a robust apparatus.

The invention further pertains to a method for transferring a patternonto a target exposure surface with an electron beam, using an electronbeam exposure apparatus described above, and to a wafer processed usingthe apparatus of the current invention. The apparatus can furthermore beused for the production of mask, like for instance used instate-of-the-art optical lithography systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further elucidated in the following embodiments ofan electron beam exposure apparatus according to the current invention,in which:

FIG. 1 shows an apparatus according to the present invention,

FIG. 2A shows a detail of a known electron beam exposure apparatus,

FIG. 2B shows a detail of the electron beam exposure apparatus,

FIG. 3 shows an electron source with a spherical outer surface,

FIG. 3A shows a source with a space charge region,

FIG. 4 shows an embodiment of a electron beam exposure apparatusstarting from the beamlets,

FIG. 5A, 5B show embodiments of scan deflection arrays of the currentinvention,

FIG. 6A, 6B show scan trajectories of the present invention,

FIG. 7A–7D show adjustment of modulation timing, and

FIG. 8A, 8B show effects of adjustment of modulation timing.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention is schematically shown in FIG. 1.Electrons are emitted from a single, stable electron source 1. Anillumination system focuses and collimates the emitted electron beam 5to illuminate a desired area on an aperture plate 6 uniformly. This canfor instance be established by using lenses 3 and 4. Due to the apertureplate 6 the electron beam 5 is split in a plurality of electronbeamlets, two of which 5a and 5b, are shown. An alternative way tocreate a plurality of electron beamlets is to use an array of electronsources. Each electron source generates an electron beamlet, which ismodulated in the same way as the one created with a combination of asingle source and splitting means. Since the emission characteristics ofeach source are slightly different, a single source 1 with beamsplitter6 is preferred. An array of electrostatic lenses 7 focuses each beamletto a desired diameter. A beamlet blanker array 8 is positioned in such away that each individual beamlet coincides with an aperture in the plateof beamlet blanker array 8. The beamlet blanker array 8 comprisesbeamlet-blankers, for instance blanking electrostatic deflectors. When avoltage is applied on a blanking deflector an electric field across thecorresponding aperture is established. The passing electron beamlet, forexample beamlet 9, deflects and terminates at the beamlet stop array 10,located behind the beamlet blanker array 8 following the electronbeamlet trajectory. When there is no voltage applied to the blankingdeflector the electron beamlet will pass the beamlet stop array 10, andreach the focusing electron optical system comprising an array ofelectrostatic lenses 13. This array 13 focuses each of the transmittedbeamlets 12 individually on the target exposure surface 14. Finallyscanning deflection means, most often electrostatic scan deflectors,move the beamlets together in one direction over the target exposuresurface 14. In the embodiment shown in FIG. 1 the scan deflectors arelocated on the target exposure surface side 11a of beamlet stop array10, thus forming an additional scan deflection array 11. However, otherlocations are also possible. During the scanning the target exposuresurface 14 and the scan deflectors moves relatively to one another in adirection different from the direction of the scan deflection. Usuallythe target is a wafer or a mask covered with a resist layer.

A remarkable aspect of the configuration shown in FIG. 1 is that theentire image that is created by the combination of beamlet blanker array8 and beamlet stop array 10 is not demagnified as a whole. Instead, eachindividual beamlet is individually focused on the target exposuresurface 14 by the focusing electron optical system 13. The differencebetween these two approaches is shown in FIGS. 2A and 2B. In FIG. 2A anentire image comprising 2 electron beamlets 5a and 5b is demagnified toacquire the desired resolution. To demagnify an image requires at leastone crossing X. In this crossing, all the electrons have to pass a smallarea. Coulomb interactions deteriorate the resolution at that crossingX.

In the present invention the method shown in FIG. 2B is used. Considertwo adjacent beamlets 5a, 5b that are projected on the target exposuresurface 14. Using the demagnification approach the distance between thetwo beamlets also becomes smaller. The focusing approach of the currentinvention, however, does not change this distance between two beamlets.Only the cross section of each beamlet is reduced.

The electron source 1 of FIG. 1 typically delivers 100 A/cm² from anarea of about 30–300 micron squared. In an embodiment, a thermionicsource is used. The electrons are preferably emitted in the space chargelimited emission regime in order to benefit from a homogenizing effectof the space charge. Examples of such a source are a LaB₆ crystal, adispenser source comprising Barium Oxide, or a dispenser sourcecomprising a layer of Barium or Tungsten covered with Scandium Oxide.

The extractor electrodes 2 usually, but not necessarily, focus the beam.The illumination lenses 3–4 create a parallel beam of electrons 5 on theaperture array 6. The lenses 3–4 are optimised to limit the beam energyspread as a result of Coulomb interactions, i.e. the opening angle ofthe beam is made as large as possible. Furthermore lenses 3–4 areoptimised to limit the beam blur created by chromatic and sphericalaberration effects. For the latter it may be advantageous to use theaperture array 6 as a lens electrode, because this may create negativechromatic and spherical aberrations, resulting in a compensation of theaberrations of lenses 3–4. Furthermore, it is possible to use lens 4 formagnification of the pattern by slightly focusing or defocusing it.

In such an embodiment, however, the electron beam emitted from thesingle emitter is focussed in a small crossover x before it is expanded.Within this crossover x there is a large energy spread due toelectron-electron interactions in this crossover x. In the end thecrossover x will be imaged demagnified on the target exposure surface.Due to the Coulomb interactions the desired resolution is not achieved.A method to expand and collimate the expanded beam without a crossoveris therefore desirable.

In a first embodiment, shown in FIG. 3, crossover in the illuminationelectron optics is avoided by using an electron source 1 with aspherical or a hemispherical outer surface 15. In this configuration alarge opening angle α is formed, which reduces the blur due toelectron-electron interactions in the emitted electron beam 5.Additionally the electron beams are forming a spherical wave front,which results in a virtual crossover 16 located in the centre of thesource. There are no electrons present in the virtual crossover; sodisturbing electron-electron interactions are absent.

The electrons can be extracted with a spherical extractor that compriseslarge holes. The main advantage of the spherical shape of the extractoris the more homogeneous field that is created.

In an alternative embodiment, shown in FIG. 3A, crossover is avoided byextracting the electrons from the source/cathode 1 which is at a voltageVs and has a distant planar extractor 11. The planar extractor has apositive voltage +V₁ with respect to the source 1. The combination ofsource and extractor now serves as a negative lens. The extractedelectrons passing the extractor 1 ₁ thus expand due to the divergingelectric field. Again, a virtual crossover is created, which reduces theloss of resolution due to Coulomb interactions to a great extent.Between source 1 and extractor 1 ₁ a space charged region S is presentas is shown in FIG. 3A. The presence of this space charge enhances thenegative lens effect created by the source-extractor combination.

By tuning V₁ it is possible to let the source 1 operate in its spacecharge limited emission mode. The main advantage of this emission modeis the significant increase of homogeneity of the emission. The increaseof the total current can be limited by selecting a source with aconfined emission area.

The aperture array 6 has apertures of typically 5–150 μm in diameterwith a pitch of about 50–500 μm. The apertures are preferably arrangedin a hexagonal pattern. The aperture array 6 splits the incomingparallel beam of electrons 5 in a plurality of electron beamlets,typically in the order of about 5,000–30,000. The size of the aperturesis adjusted to compensate non-uniform current density of theillumination. Each aperture has an area inversely proportional to thecurrent density based on the individual beamlets that is transmittedthrough that same aperture. Consequently the current in each individualbeamlet is the same. If the heat load on the aperture plate becomes toolarge, several aperture arrays are arranged in a serial order withdecreasing aperture diameters along the path of the electron beam orplurality of electron beamlets. These aperture arrays have mutuallyaligned apertures.

Another possible way to split the collimated electron beam 5 into aplurality of electron beamlets is the use of a quadrupole lens array. Apossible configuration of such an array is disclosed in U.S. Pat. No.6,333,508, which document is referenced here as if fully set forth.

FIG. 4 shows a detail closer image of the lithography system in one ofthe embodiments of the present invention starting from the plurality ofbeamlets. Condensor lens array 7 focuses each beamlet to a diameter ofabout 0.1–1 μm. It comprises two aligned plates with holes. Thethickness of the plates is typically about 10–500 μm, while the holesare typically about 50–200 μm in diameter with a 50–500-μm pitch.Insulators (not shown), which are shielded from the beamlets, supportthe plates at typical distances of 1–10 millimetres from each other.

The modulation array comprises a beamlet blanker array 8 and a beamletstop array 10. At the beamlet blanker array 8, the typical beam diameteris about 0.1–5 μm while the typical transversal energy is in the orderof a 1–20 meV. Beamlet blanking means 17 are used to switch the electronbeamlets on and off. They include blanking electrostatic deflectors,which comprise a number of electrodes. Preferably at least one electrodeis grounded. Another electrode is connected to a circuit. Via thiscircuit control data are sent towards the blanking electrostaticdeflectors. In this way, each blanking deflector can be controlledindividually. Without the use of the beamlet blanking means 17 theelectron beamlet will pass the beamlet stop array 10 through theapertures. When a voltage is applied on a blanking electrostaticdeflector electrode in the beamlet blanker array 8, the correspondingelectron beamlet will be, deflected and terminate on the beamlet stoparray 10.

In an embodiment, the beamlet blanker array 8 is located in theelectrostatic focal plane of the electron beamlets. With the blankerarray in this position, the system is less sensitive for distortions. Inthis embodiment, the beamlet stop array is positioned outside a focalplane of the electron beamlets.

The transmitted beamlets now have to be focused on the target exposuresurface 14. This is done by a focusing electron optical system 13comprising at least one array with electrostatic lenses. Eachindividually transmitted electron beamlet is focused on the targetexposure surface by a corresponding electrostatic lens. The lens arraycomprises two or more plates 13a and 13b, both having a thickness ofabout 10–500 μm and apertures 13c with a diameter of about 50–250 μm.The distance between two consecutive plates is somewhere between 50-800μm and may be different from plate to plate. If necessary, the focusingelectron optical system may also comprise a lens array of the magnetictype. It is then located between the beamlet stop array 10 and theobjective lens array of the electrostatic type 13, to further enhancethe focusing properties of the electron optical system.

A major problem in all electron beam lithography systems patterning awafer or a mask is contamination. It reduces the performance of thelithography system significant due to the interaction between electronsand particles in the resist layer, the resist degrades. In a polymericresist, molecules are released due to cracking. The released resistparticles travel through the vacuum and can be absorbed by any of thestructures present in the system.

In order to cope with the contamination problem, in a particularembodiment protective means are located in close proximity of the targetexposure surface, i.e. between the target exposure surface and thefocusing electron optical system. Said protective means may be a foil ora plate. Both options are provided with apertures with a diametersmaller than 20 μm. The protective means absorb the released resistparticles before they can reach any of the sensitive elements in thelithography system. In some cases it is necessary to refresh theprotective means after a predetermined period, e.g. after everyprocessed wafer or mask. In the case of a protective plate the wholeplate can be replaced. In a particular embodiment, the foil is woundaround the coil winders. A small section of the foil is tightened justabove the entire target exposure surface 14. Only this section isexposed to the contaminants. After a certain period the protectivecapacity of the foil rapidly degrades due to the absorbed particles. Theexposed foil section then needs to be replaced. To do this the foil istransported from one coil winder to the other coil winder, thus exposinga fresh foil section to the contamination particles.

The entire system that is described above operates at relatively lowvoltages. In operations in which high-energy electrons are needed, anadditional acceleration stage is positioned between the electrostaticlens array of the focusing electron optical system 13 and the protectivemeans. This acceleration stage adds energy to the passing electrons. Thebeam may be accelerated additional tens of kiloelectronvolts, e.g. 50keV.

As explained earlier in FIG. 1, the beamlets 12 that have successfullypassed the beamlet stop array 10 are directed towards the desiredposition on the target exposure surface 14 by two means. First of allactuation means move the target exposure surface 14 and the rest of thesystem in a certain mechanical scan direction relatively to each other.Secondly scan deflection means scan the transmitted beamlets 12electrostatically in a direction that differs from the mechanical scandirection. The scan deflection means comprise electrostatic scandeflectors 18. In FIGS. 1 and 3 these scan deflectors 18 are located onan additional aperture array 11, and are depicted in FIG. 4.

In one embodiment, the electrostatic scan deflectors 18 are deposited onthe target exposure surface side of one of the plates of the objectiveelectrostatic lens array 13, such that the deflection essentially occursin the front focal plane of the objective lenses. The desired result isthat the deflected beamlets impinge perpendicularly on the targetsurface.

In another embodiment there are two deflector arrays, one deflecting ina first direction and the other deflecting in a second, oppositedirection. The combined deflection causes displacement of the beamlets adisplacement of the beamlets at the target surface location, withoutchanging the perpendicular axis of a beamlet with respect to the targetsurface.

In a second embodiment, the electrostatic scan deflectors 18 are locatedon the protective means.

The electrostatic scan deflectors 18 comprise scan deflectionelectrodes, which are arranged to deflect an assembly of electronbeamlets in the same direction. The scan deflection electrodes may bedeposited in the form of strips 19 on a suitable plate 20 at the targetexposure surface side as is shown in FIG. 5A. The best yield can beestablished when the strips 19 are deposited close to the beamlet, thusclose to the aperture 21, since this reduces d_(b-sd). Moreover, it ispreferable to position the scan deflection electrodes outside anindividual beamlet crossover plane.

In one embodiment the first assembly is scanned in one direction whilethe next one is scanned in the opposite direction, by puttingalternating voltages on the consecutive strips 19 as is shown in FIG.5B. The first strip has for instance a positive potential, the secondone a negative potential, the next one a positive etc. Say the scandirection is denoted y. One line of transmitted electron beamlets isthen scanned in the −y-direction, while at the same time the next lineis directed towards +y.

As already mentioned there are two scan directions, a mechanical scandirection M and a deflection scan direction S, both depicted in FIGS. 6Aand 6B. The mechanical scan can be performed in three ways. The targetexposure surface moves, the rest of the system moves or they both movein different directions. The deflection scan is performed in a differentdirection compared to the mechanical scan. It is preferablyperpendicular or almost perpendicular to the mechanical scan direction,because the scan deflection length Δx is then larger for the samedeflection scan angle α_(sd). There are two preferable scantrajectories, both shown in FIG. 6 for clarity. The first one is atriangular shaped scan trajectory (FIG. 6A), the second one a saw toothshaped scan trajectory (FIG. 6B).

When the mechanical scan length is a throughput-limiting factor, anassembly of electron beam exposure apparatuses as described above isused to expose the entire wafer at the same time.

It is assumed that an ideal grid exists on the wafer and that theelectron beamlets can be positioned exactly on the grid coordinates. Saythat a correct pattern is created when the electron beamlet can bepositioned within 1/30^(th) of the minimum feature size. Then to writeone pixel, 30 scan lines and thus 30*30=900 grid points are needed. Forthe 45 nm-mode the positioning should be controllable within a range of1.5 nm. The data path should therefore be able to handle an enormousamount of data.

The writing strategy described above is based on the assumption that thebeamlet can only be switched on or off. To reduce the amount of data byless grid lines, and thus less grid cells seems a logical approach.However, the dimension control of the desired pattern suffersconsiderably. An approach to circumvent this problem is to pattern thetarget exposure surface 14 with discrete dose control. Again the patternis divided according to a rectangular grid. However, the number of gridlines is much smaller e.g. 2–5 per dimension, which results in a numberof grid points of about 4–25. In order to get the same patternreliability as for the finer grid, the intensity of each grid cell isvariable. The intensity is represented by a so-called gray value. Incase of a 3 bit gray value representation, the values are 0, 1/7, 2/7,3/7, 4/7, 5/7, 6/7 and 1 times the maximum dose. The number of datarequired for the position of the beamlet reduces, although each cell isrepresented with more information due to the controlled dose variation.

In the present invention gray scale writing can be introduced in severalways. First of all the deflection of the beams may be controlled in sucha way that part of the beam passes the beamlet stop array 10, while partof the beam continues traveling towards the target exposure surface 14.In this way for instance ⅓ or ⅔ of the beam can be stopped, resulting in4 possible doses on the target exposure surface, namely 0, ⅓, ⅔ and 1times the maximum dose, corresponding to a 2 bit gray valuerepresentation.

Another method to create gray levels is to deflect the beamlets in sucha way that they do not move with respect to the target surface for apredetermined amount of time T, which amount of time T is longer than aminimum on/off time of the blankers. During time T, the modulator cannow deposite 1, 2, 3, etc. shots on one position, thus creating graylevels.

Another method to create these 4 so-called gray values is to change theaperture size in the aperture array 6. If there are for instance threeaperture sizes, the original size, a size that permits half the originalcurrent to pass and apertures with an area such that only a fourth ofthe original current passes, the same discrete dose values as mentionedbefore an be created. By switching the beamlets on and off with thedeflection electrodes 17 of the beamlet blanker array 8 the desired dosecan be deposited on the target exposure surface 14. A disadvantage ofthe latter method is the fact that more beamlets are needed to write onepixel. Most, including aforementioned methods for discrete dose controlcan also be used to create more than 4 gray values, e.g. 8, 16, 32 or64.

The positions of the beamlets on the target exposure surface most oftendo not exactly correspond with the desired positions. This is forinstance due to misalignment of the different arrays with respect toeach other. Additionally, manufacturing errors may also contribute tothe offset of the individual beamlets. To transfer the correct patternfrom the controller onto the exposure surface of the target, correctionshave to be made. To this end, in a particular embodiment, first theposition of all beamlets is measured and stored. Each position is thencompared to the position the beamlet should have. The difference inposition is then integrated in the pattern information that is sent tothe modulation means.

Since changing the signal sequence that is sent towards the modulationmeans takes a lot of time, the measured difference in position isintegrated in the pattern information by transforming it into acorresponding difference in timing in the beamlet modulation control.FIGS. 7A–7D and 8A–8B explain how the adjustments are implemented. Asalready mentioned the beamlet scan is performed by combining two scanmechanisms: a mechanical scan and a deflection scan. All pattern data,which is sent to each beamlet, is supplied per deflection scan line. Thedesired deflection scan width on the exposure surface of the target thatis patterned, W_(scan), is smaller than the deflection scan width theapparatus can handle, W_(overscan), as is shown in FIGS. 7A AND 7B. Theoverscan ability enables a correction in the deflection scan direction.In FIG. 7A the beamlet is positioned correctly. In FIG. 7B, however, thebeamlet has shifted to the right. By adjusting the timing in such a waythat the pattern data is applied when the beamlet enters the desiredarea, the offset can be compensated for. The adjustment in themechanical scan direction is less precise than depicted in FIG. 7B.Since the pattern data is written per scan line, only a discrete timedelay is possible, i.e. pattern generation can be postponed oraccelerated per scan line. A random time delay would result in acompletely new control data sequence. A calculation of such a newsequence takes a lot of time and is therefore not desirable. In FIGS. 7CAND 7D is depicted what the consequence is. In FIG. 7C again the desiredlocation of the beamlet is shown together with its first fivecorresponding scan lines. In FIG. 7D the real position of the beamletand its trajectories is shown. For clarity the desired beamlet and scanlines are also depicted with an empty circle and dashed lines,respectively. It can be seen that the first scan line in the desiredsituation does not cover the area that needs to be patterned by thebeamlet. So the beamlet start patterning halfway the second scan line.Effectively the delay of information has taken a time period that isnecessary to scan one deflection scan line.

FIGS. 8A and 8B show an example of how a change in the timing correctsfor the initial incorrect position of a structure written by a notideally positioned beamlet. FIG. 8A depicts the situation without anytiming correction. The empty dot represents the beamlet at the correctposition, while the filled one represents the real location of thebeamlet. The beamlet is scanned along the drawn line to write a pattern.The line is dashed in the ideal case and solid in the real case. In thisexample the written structure is a single line. Consider a black andwrite writing strategy, i.e. the beamlet is “on” or “off”. The patternis written when the “on” signal is sent towards the modulation means. Inorder to write the single line a certain signal sequence like the oneshown in the upper curve is sent towards the modulation means. When thesame signal sequence is sent in reality, the line is written at adifferent position than desired. The offset of the beamlet leads to anoffset of the written structure.

FIG. 8B shows the situation wherein timing correction is applied. Againthe theoretical and actual spots and trajectories are depicted withdashed and solid lines and dots respectively. The signal sequence in thereal situation is different than the theoretical pattern information, inthe fact that the signal sequence in the real situation (lower curve) issent at a different time than the same sequence is sent in the ideaconfiguration (upper curve). As a result the single line is now writtenat the correct location in the deflection scan direction. Moreover thepattern processing started one scan line earlier resulting in a betterpositioning of the single line in the mechanical scan direction as well.Note that the single line is not precisely positioned at the correctlocation. This is due to the slight offset between the scan lines in theideal and the real situation.

The current electron beam exposure system is thus capable of dynamicallyadjusting the position of a scanned line using timing corrections. Thisallows for critical components in a pattern to be written in one scanline instead of using two halves of two scan lines, which would spreadthe critical component over two scan lines. This correction can also bedone locally, i.e. the timing can be corrected over a small time window.The controller should thus identify critical components, which wouldnormally be spread over two scan lines. Subsequently, the controllershould calculated a corrected timing window, and apply the correctedtiming window to the timing base used for scanning an electron beamlet.FIG. 7D shows the adjustment principle, which could be used for this.

All lens plates, aperture plates and blanker plates can be connected toa power supply, which, when gas is admitted into the system, creates aplasma. The plasma cleans the plates and removes all contamination. Ifone plasma does not clean thorough enough, two gases may be admittedinto the system in series. For instance oxygen may be admitted first toremove all hydrocarbons residing in the system. After the removal of theoxygen plasma, a second plasma, for instance comprising HF, is createdto remove all present oxides.

Another possibility to reduce the contamination is to perform alloperations at elevated temperatures, i.e. 150–400° C. A pretreatment at1000–1500° C. may be necessary. At these temperatures hydrocarbons getno chance to condense on any of the elements in the system. Allowing afraction of oxygen into the system can further enhance the cleaningprocess.

It is to be understood that the above description is included toillustrate the operation of the preferred embodiments and is not meantto limit the scope of the invention. The scope of the invention is to belimited only by the following claims. From the above discussion, manyvariations will be apparent to one skilled in the art that would yet beencompassed by the spirit and scope of the present invention.

The invention claimed is:
 1. A method for transferring a pattern onto atarget exposure surface with a multi-beam lithography system, comprisingthe steps of: generating a plurality of beamlets; individuallymodulating the intensity of each beamlet of said plurality of beamletsby means of a modulator device for blanking or not blanking each beamletin whole or in part; controlling said modulator device, using controlsignals, by means of a controller operationally coupled to saidmodulator; and individually adjusting at least one of said controlsignals.
 2. The method according to claim 1 in which timing of saidcontrol signals is adjusted.
 3. The method of claim 1, in which timingof said control signals is individually adjusted.
 4. The methodaccording to claim 1 in which modulation is performed as an “off” or“on” condition of a beamlet, by either deflecting said beamlet withinthe system, or by allowing free passage of said beamlet at saidmodulator, the condition thereby being controlled on the basis ofavailable electronic pattern data, in which timing is adjusted bycorrecting an instance of deflection or passage of said beamlet, saidcorrection being calculated by the controller.
 5. The method of claim 1,wherein said control signals having have a timing base, and timing ofthe control signal of at least one beamlet is adjusted.
 6. The method ofclaim 1, further comprising the step of determining a position of abeamlet, storing said position in a memory and comparing said positionwith a desired position.
 7. The method of claim 6, wherein said positionof a beamlet is the actual position of a beamlet on said exposuresurface.
 8. The method of claim 6, wherein said adjustment of saidtiming is based on the result of said comparing.
 9. The method of claim1 2, wherein said timing is adjusted locally.
 10. The method of claim 12, wherein said adjusting of timing of said control signals comprisescorrecting a timing window.
 11. The method of claim 1 2, wherein saidcontrol signals have a timing base.
 12. The method of claim 11, whereinsaid controller calculates a corrected timing window, and applies saidcorrected timing window to said timing base.
 13. The method of claim 11,wherein said controller calculates a corrected timing window, andapplies said corrected timing window to said timing base of anindividual beamlet.
 14. An electron beam exposure apparatus fortransferring a pattern onto the surface of a target using a plurality ofelectron beamlets, comprising: a modulator array for receiving saidplurality of electron beamlets, comprising a plurality of modulators formodulating the intensity of a beamlet of said plurality of beamlets; acontroller, operationally coupled to said modulator array, forcontrolling each modulator of said plurality modulators on the basis ofelectronic pattern data to write the pattern, said controller producinga plurality of control signals with at least one control signal for eachmodulator, and an adjustor for allowing individual adjustment of acontrol signal.
 15. The electron beam exposure apparatus according toclaim 14, wherein said plurality of control signals having have a timingbase.
 16. The electron beam exposure apparatus according to claim 14,wherein said adjustor is adapted for allowing individual adjustment oftiming of a control signal.
 17. The electron beam exposure apparatusaccording to claim 14, furthermore provided with a measuring device formeasuring the actual position of at least one of said beamlets, andwherein the controller is provided with a memory for storing said actualposition and a desired position, a comparator for comparing the desiredposition and the actual position of said at least one of said beamlets,and wherein the adjustor is operationally coupled to the controller forreceiving instructions for adjusting a control signal issued to amodulator to compensate for the difference between said desired positionand said actual position of said at least one of said electron beamlets.18. The electron beam exposure apparatus according to claim 14, whereinthe adjustor is operationally coupled to the controller for receivinginstructions indicating the amount of the adjustments.
 19. The electronbeam exposure apparatus of claim 14, wherein the adjustor is adapted foradjusting timing of each control signal.
 20. The electron beam exposureapparatus of claim 14, further comprising a beamlet generator, saidbeamlet generator comprising: a source for emitting at least oneelectron beam, and at least one beamsplitter for splitting said at leastone emitted electron beam into said plurality of electron beamlets. 21.The electron beam exposure apparatus according to claim 20, furthercomprising a modulation array, comprising a beamlet blanker arraycomprising a plurality of beamlet blankers for the deflection of apassing electron beamlet and a beamlet stop array, having a plurality ofapertures aligned with said beamlet blankers of said beamlet blankerarray.
 22. The electron beam exposure apparatus according to claim 2021, further comprising a second electrostatic lens array located betweensaid beamsplitter and said beamlet blanker array to focus said pluralityof electron beamlets.
 23. The electron beam exposure apparatus accordingto claim 22, wherein said beamlet blanker array is located in the focalplane of said second electrostatic lens array.
 24. An electron beamexposure apparatus for transferring a pattern onto the surface of atarget using a plurality of electron beamlets, comprising: a modulatorarray for receiving said plurality of electron beamlets, comprising aplurality of modulators for modulating the intensity of a beamlet ofsaid plurality of beamlets; and a controller, operationally coupled tosaid modulator array, for controlling each modulator of said pluralitymodulators on the basis of electronic pattern data to write the pattern,said controller producing a plurality of control signals with at leastone control signal for each modulator, said controller comprising anadjustor allowing individual adjustment of at least one control signal.25. The electron beam exposure apparatus of claim 24, wherein saidplurality of control signals having have a timing base.
 26. The electronbeam exposure apparatus of claim 24, said adjustor allowing individualadjustment of timing of at least one control signal.
 27. An electronbeam exposure apparatus for transferring a pattern onto the surface of atarget using a plurality of electron beamlets, comprising: a modulatorarray comprising a plurality of modulators for modulating the intensityof a beamlet of said plurality of beamlets; a scanning deflector forscanning said plurality of electron beamlets over said surface of saidtarget, comprising at least one array of electrostatic deflectors havingat least one electrostatic deflector for each beamlet; a controller,operationally coupled to said scanning deflector, for controlling eachelectrostatic deflector individually using a control signal; and anadjustor for adusting adjusting said control signal.
 28. The electronelectron beam exposure apparatus of claim 27, wherein said controlsignal has a timing base, and said adjustor being adapted for adjustingtiming of said control signal.
 29. An electron beam exposure apparatusfor transferring a pattern onto the surface of a target using aplurality of electron beamlets, comprising: a scanning deflector forscanning said plurality of electron beamlets over said surface of saidtarget; a beamlet blanking array, comprising at least one array ofelectrostatic deflectors having at least one electrostatic deflector foreach beamlet for blanking beamlets on the basis of electronic patterndata; a controller, operationally coupled to said scanning deflectorbeamlet blanking array, for controlling each electrostatic deflectorusing a control signal, said controller being adapted for adjusting atleast one of the control signal signals.
 30. The electron beam exposureapparatus of claim 29, wherein said controller is operationally coupledto said scanning deflector beamlet blanking array for controlling eachelectrostatic deflector using a control signal having a timing base. 31.The electron beam exposure apparatus of claim 29, wherein saidcontroller being adapted for adjusting timing of at least one controlsignal so that said pattern data is applied when the correspondingbeamlet enters a desired area to be written by the beamlet.
 32. Theelectron beam exposure apparatus of claim 29, wherein said controllergenerates a plurality of control signals, at least one control signalfor each electrostatic deflector.
 33. The electron beam exposureapparatus of claim 32, wherein said controller is adapted for adjustingtiming of each control signal.
 34. The electron beam exposure apparatusof claim 32, wherein said controller is adapted for adjusting timing ofeach control signal individually.
 35. An electron beam exposureapparatus for transferring a pattern onto the surface of a target usinga plurality of electron beamlets, comprising: a blanking deflectionmeans for effectively realising an off/on condition for individualbeamlets at said target surface, by deflecting such individual beamletswithin the system; a controller, operationally coupled to said blankingdeflection means, for individually controlling electrostatic deflectorsof said blanking deflection means, said controller using an individualcontrol signal for each deflector of said deflection means, therebydetermining said on and off condition for individual beamlets;correction means, part of, or operationally associated with saidcontroller for individually correcting a timing window of a beamletadjusting the control signal of the blanking deflector for said beamlet.36. An electron beam exposure apparatus for transferring a pattern ontothe surface of a target using a plurality of electron beamlets,comprising: a blanking deflection device for effectively realising anoff/on condition for individual beamlets at said target surface,comprising a plurality of deflectors for deflecting such individualbeamlets within the system for realising said off/on condition; acontroller, operationally coupled to said blanking deflection device,for individually controlling electrostatic deflectors of said blankingdeflection device, said controller using an individual control signalfor each deflector of said deflection device, thereby determining saidon and off condition for individual beamlets; a correction device, partof, or operationally associated with said controller for individuallycorrecting a timing window of a beamlet adjusting the control signal ofthe blanking deflector for said beamlet.
 37. The method according toclaim 4, wherein said modulation is controlled on the basis of availableelectronic pattern data in which timing is adjusted by correcting aninstance of deflection or passage of said beamlet, said correction beingcalculated by the controller.
 38. A maskless lithography system fortransferring a pattern onto the surface of a target, comprising: anelectron beam generator for generating an electron beam; an opticalsystem and beam splitter for generating a plurality of separate beamletsfrom the electron beam; a modulation array for modulating individualbeamlets on the basis of electronic pattern data to write the pattern;and a focusing electron optical system for projecting the beamlets ontothe target and reducing the cross section of the individual beamlets;wherein said beamlets projected onto the target are maintained separateto each other until projection of said beamlets on to said target. 39.The system according to claim 38, wherein said system further includes,downstream of said beam splitter, a condensor lens array for focusingindividual beamlets within said plurality of beamlets.
 40. The systemaccording to claim 39, wherein said individual beamlets are focused to adiameter in a range from about 0.1 to 1 μm.
 41. The system according toclaim 39, wherein the condensor lens array comprises two aligned plateswith holes.
 42. The system according to claim 41, wherein the thicknessof the plates is within a range from about 10 to 500 μm.
 43. The systemaccording to claim 41, wherein the condensor lens array comprises aplate with holes of a diameter within a range from 50 to 200 μm, and apitch within a range from about 50 to 500 μm.
 44. The system accordingto claim 41, wherein the system further comprises insulators forsupporting said plates.
 45. The system according to claim 38, whereinsaid modulation array includes a beamlet blanker array and a beamletstop array.
 46. The system according to claim 45, wherein saidmodulation means is included between said beam splitter and saidprojection lenses.
 47. The system according to claim 45, wherein thebeam diameter is within the range from about 0.1 to 5 μm at the beamletblanker array.
 48. The system according to claim 45, wherein thetransversal energy at said beamlet blanker array is within the rangefrom about 1 to 20 meV.
 49. The system according to claim 45, whereinthe beamlet blanker array comprises an array of electrostaticdeflectors, each electrostatic deflector comprising a first electrodeconnected to ground and a second electrode connected to a circuitreceiving control data.
 50. The system according to claim 49, whereineach electrostatic deflector is controlled individually.
 51. The systemaccording to claim 45, wherein the beamlet blanker array is located inan electrostatic focal plane of the plurality of beamlets.
 52. Thesystem according to claim 51, wherein the beamlet stop array ispositioned outside an electrostatic focal plane of the plurality ofbeamlets.
 53. The system according to claim 38, wherein the focusingelectron optical system comprises an array of electrostatic lenses forfocusing each individual beamlet within said plurality of beamlets witha corresponding electrostatic lens.
 54. The system according to claim53, wherein the array of electrostatic lenses comprises two or moreplates, each plate having a thickness within a range from about 10 to500 μm.
 55. The system according to claim 53, wherein the array ofelectrostatic lenses comprises two or more plates, the distance betweenconsecutive plates being within a range from 50 to 800 μm.
 56. Thesystem according to claim 53, wherein the array of electrostatic lensescomprises three or more plates, the distance between consecutive platesbeing different from plate to plate.
 57. The system according to claim38, wherein the focusing electron optical system comprises a lens arrayof the magnetic type.
 58. The system according to claim 45, wherein thefocusing electron optical system comprises a lens array of the magnetictype and an array of electrostatic lenses, the lens array of themagnetic type being located between the beamlet stop array and the arrayof electrostatic lenses.
 59. The system according to claim 57, whereinthe lens array of the magnetic type is included for enhancing thefocusing properties of the projection system.
 60. The system accordingto claim 38, wherein the beam splitter is formed by a spatial filter.61. The system according to claim 58, wherein the spatial filter isformed by an aperture array.
 62. The system according to claim 61,wherein the apertures of the aperture array are arranged in a hexagonalstructure.
 63. The system according to claim 61, wherein each apertureof the aperture array has an area inversely proportional to the currentdensity of a beamlet that is, in use, transmitted through said aperture.64. The system according to claim 38, wherein the beam splittercomprises an electrostatic quadruple lens array.
 65. The systemaccording to claim 38, wherein the beam splitter comprises a number ofaperture arrays in series along the path of the electron beam orplurality of beamlets, the aperture arrays having mutually alignedapertures, each subsequent aperture array along the path from electronbeam generator to target having apertures being smaller thancorresponding apertures of the previous array.
 66. The system accordingto claim 38, wherein, along a beamlet path from electron beam generatorto target, an electrostatic lens array is located immediately after saidbeam splitter.
 67. The system according to claim 38, the system furthercomprising: a beamlet blanker array for controllably deflectingindividual beamlets of said plurality of beamlets; and a beamlet stoparray for obstructing deflected beamlets and letting through undeflectedbeamlets.
 68. The system according to claim 67, wherein the beamletblanker array is located in a focal plane of an electrostatic lens arrayfor focusing said plurality of beamlets.
 69. The system according toclaim 38, wherein said beamlets projected onto the target are maintainedparallel to one another.
 70. A deflection system for deflecting aplurality of electron beamlets with respect to a target surface, thedeflection system comprising: a first deflector array for deflecting thebeamlets in a first direction; and a second deflector array fordeflecting the beamlets in a second direction, the second directionbeing opposite to the first direction.
 71. The deflection systemaccording to claim 70, wherein, in use, combined deflection of a beamletby said first deflector array and said second deflector array results indisplacement of the beamlet at the target surface without changing anorientation of the beamlet with respect to the target surface.
 72. Thedeflection system according to claim 71, wherein said orientation of thebeamlet is perpendicular to the target surface.
 73. A masklesslithography system for transferring a pattern onto a surface of atarget, comprising: a beamlet generator for generating a plurality ofelectron beamlets; a modulation array for modulating an intensity ofelectron beamlets of the plurality of electron beamlets; a deflectionsystem according to claim 70 for deflecting the modulated electronbeamlets; a target holder for holding a target having a surface forreceiving the pattern to be transferred.
 74. The system according toclaim 73, wherein the system further comprises an electron opticalsystem for focusing the modulated electron beamlets on the surface ofthe target.
 75. The system according to claim 73, wherein the deflectionsystem is positioned within the electron optical system such thatdeflection occurs in a front focal plane of the electron optical system.76. An electron-optical arrangement for use in a maskless lithographysystem, the arrangement comprising: a deflector system according toclaim 70 for deflecting a plurality of beamlets; and an electrostaticlens array for focusing said plurality of deflected beamlets; whereinthe deflector array is positioned in a front focal plane of theelectrostatic lens array.
 77. The arrangement according to claim 76,wherein the electrostatic lens array comprises two or more plates, andthe deflector array is formed by deposition of electrostatic scandeflectors on a target surface side of one of the two or more plates ofthe electrostatic lens array.
 78. The arrangement according to claim 76,wherein the electrostatic lens array comprises two or more plates, eachplate having a thickness within a range from about 10 to 500 μm.
 79. Thearrangement according to claim 76, wherein the electrostatic lens arraycomprises two or more plates, the distance between consecutive platesbeing within a range from about 50 to 800 μm.
 80. The arrangementaccording to claim 76, wherein the electrostatic lens array comprisesthree or more plates, the distance between consecutive plates beingdifferent from plate to plate.
 81. A method of displacement of aplurality of beamlets with respect to a target surface, the methodcomprising: deflecting the beamlets in a first direction by means of afirst deflector array; deflecting the beamlets in a second, oppositedirection by means of a second deflector array.
 82. The method accordingto claim 81, wherein the combined deflection in the first direction andthe second direction results in displacement of the beamlet at thetarget surface without changing an orientation of the beamlet withrespect to the target surface.
 83. The method according to claim 82,wherein the orientation of the beamlet after the combined deflection isperpendicular to the target surface.
 84. The method according to claim81, wherein the plurality of beamlets are parallel with respect to eachother before the first deflecting step and after the second deflectingstep.
 85. The method according to claim 81, further comprising providingan electrostatic lens array positioned so that the deflector arrays arein a front focal plane of the electrostatic lens array; and focusing theplurality of beamlets with the electrostatic lens array.
 86. A masklesslithography system for transferring a pattern onto the surface of atarget, comprising: an electron beam generator for generating anelectron beam; a beam splitter for splitting the electron beam into aplurality of electron beamlets; a modulation array for modulating theplurality of beamlets on the basis of electronic pattern data inaccordance with the pattern to be transferred; an array of electrostaticscan deflectors for deflecting electron beamlets to scan the targetsurface, wherein the array of electrostatic scan deflectors comprisescan deflection electrodes, each scan deflection electrode beingarranged to deflect a group of electron beamlets in the same direction;an optical system for focusing the plurality of beamlets; a targetholder for holding a target with its surface onto which the pattern isto be transferred in a focal plane of the optical system.
 87. Themaskless lithography system according to claim 86, wherein thedeflection electrodes are electrodes deposited on a plate.
 88. Themaskless lithography system according to claim 87, wherein thedeflection electrodes are deposited on the side of the plate facing thetarget holder.
 89. The maskless lithography system according to claim86, wherein the deflection electrodes comprise strips.
 90. The masklesslithography system according to claim 86, wherein the modulation arraycomprises an array of apertures, and the deflection electrodes arepositioned close to said apertures.
 91. The maskless lithography systemaccording to claim 90, wherein the deflection electrodes are positionedin a front focal plane of the electrostatic lenses.
 92. The masklesslithography system according to claim 86, wherein the scan deflectionelectrodes comprise a first group of strips and a second group ofstrips, the first group of strips being arranged to scan in a firstdirection, the second group of strips being arranged to scan in a seconddirection.
 93. The system according to claim 86, wherein the deflectionelectrodes are in the form of strips.
 94. The system according to claim93, wherein alternating voltages are located on consecutive strips. 95.The maskless lithography system according to claim 92, wherein thedirection is opposite to the second direction.
 96. A masklesslithography system for transferring a pattern onto the surface of atarget, comprising: an electron beam generator for generating anelectron beam; an electron-optical arrangement; and a target holder forholding a target provided with a surface for receiving the pattern to betransferred; wherein the electron-optical arrangement comprises: a beamsplitter for splitting the electron beam into a plurality of beamlets; afirst electrostatic lens array for focusing the beamlets; a modulationarray comprising a plurality of modulators for modulating the beamletson the basis of electronic pattern data to write the pattern; adeflector array comprising a plurality of electrostatic deflectors fordeflecting a portion of the beamlets in a predetermined direction; asecond electrostatic lens array for focusing the deflected beamlets. 97.The maskless lithography system according to claim 96, wherein the beamsplitter comprises an aperture array having apertures arranged in ahexagonal pattern.
 98. The maskless lithography system according toclaim 96, wherein said beam splitter comprises an aperture array havingabout 5,000 to 30,000 apertures.
 99. The maskless lithography systemaccording to claim 96, wherein said beam splitter comprises an aperturearray having apertures with a size adjusted to compensate fornon-uniform current density of said electron beam.
 100. The masklesslithography system according to claim 99, wherein each aperture has anarea inversely proportional to a current density based on the respectivebeamlet to be transmitted through said aperture.
 101. The masklesslithography system according to claim 96, wherein said beam splittercomprises a number of aperture arrays in series along a path of theelectron beam or plurality of beamlets, the aperture arrays havingmutually aligned apertures, each subsequent aperture array along thepath towards the first electrostatic lens array having apertures thatare smaller than the apertures of a previous aperture array.
 102. Themaskless lithography system according to claim 96, wherein said firstelectrostatic lens array is arranged to focus individual beamlets withinthe plurality of beamlets to a diameter in a range from about 0.1 to 1μm.
 103. The maskless lithography system according to claim 102, whereinthe first electrostatic lens array comprises two aligned plates withholes.
 104. The maskless lithography system according to claim 103,wherein the thickness of the plates is within a range from about 10 to500 μm.
 105. The maskless lithography system according to claim 103,wherein the holes have a diameter within a range from about 50 to 200μm, and a pitch within a range from 50 to 500 μm.
 106. The masklesslithography system according to claim 103, wherein the firstelectrostatic lens array further comprises insulators for supportingsaid plates.
 107. The maskless lithography system according to claim 96,wherein the modulation array comprises: a beamlet blanker aperture arrayprovided with said modulators, said modulators being furtherelectrostatic deflectors for deflecting beamlets in a furtherpredetermined direction; a beamlet stop array for terminating beamletsdeflected by the further electrostatic deflectors of the beamlet blankerarray.
 108. The maskless lithography system according to claim 107,wherein the first electrostatic lens array is arranged to focusindividual beamlets within the plurality of beamlets to a diameter in arange from about 0.1 to 5 μm at the beamlet blanker array.
 109. Themaskless lithography system according to claim 107 wherein the beamletblanker array is located in an electrostatic focal plane of theplurality of electron beamlets.
 110. The maskless lithography systemaccording to claim 109, wherein the beamlet stop array is positionedoutside a focal plane of the plurality of electron beamlets.
 111. Themaskless lithography system according to claim 107, wherein each furtherelectrostatic deflector comprises a first electrode connected to ground,and a second electrode connected to a circuit for receiving controldata.
 112. The maskless lithography system according to claim 96,wherein the second electrostatic lens array comprises two or moreplates, each plate having a thickness within a range from about 10 to500 μm.
 113. The maskless lithography system according to claim 96,wherein the second electrostatic lens array comprises two or moreplates, the distance between consecutive plates being within a rangefrom 50 to 800 μm.
 114. The maskless lithography system according toclaim 96, wherein the second electrostatic lens array comprises three ormore plates, the distance between consecutive plates being differentfrom plate to plate.
 115. A method of cleaning an electron opticalsystem comprising an electron beam generator for generating an electronbeam, a target holder for holding a target on to which one or morebeamlets are to be projected, and at least one of: a beam splitter forsplitting the electron beam into a plurality of beamlets; one or moreelectrostatic lens arrays for focusing the beam or beamlets; amodulation array comprising a plurality of modulators for modulating thebeam or beamlets; a deflector array comprising a plurality ofelectrostatic deflectors for deflecting the beam or a portion of thebeamlets in a predetermined direction; and a second electrostatic lensarray for focusing the deflected beamlets; the system further comprisinga power supply connected to at least one of the beam splitter, the firstelectrostatic lens array, the modulation array, the deflector array, andthe second electrostatic lens array; wherein the method comprises:admitting a gas into the electron optical system; supplying power bymeans of the power supply to the electron optical system for creating aplasma therein; terminating said supplying of power; and removing saidgas from the electron optical system.
 116. The method according to claim115, wherein said gas comprises oxygen.
 117. The method according toclaim 116, the method further comprising: adding a further gas forremoval of oxides into the maskless lithography system; resupplyingpower to the maskless lithography system; terminating said resupplyingof power; and removing said further gas from the maskless lithographysystem.
 118. The method according to claim 117, wherein said further gascomprises HF.
 119. A method for transferring a pattern onto the surfaceof a target, comprising: generating a plurality of electron beamlets;modulating the plurality of beamlets on the basis of electronic patterndata in accordance with the pattern to be transferred; providing anarray of electrostatic scan deflectors having electrodes in the form ofstrips; deflecting the electron beamlets using the array ofelectrostatic scan deflectors to scan the target surface; and focusingthe plurality of beamlets onto the target surface onto which the patternis to be transferred.
 120. The method according to claim 119, furthercomprising: creating relative movement in a first direction between theplurality of beamlets and the target; and wherein the step of providingan array of electrostatic scan deflectors comprises providing stripsoriented in a direction corresponding to the first direction.
 121. Themethod according to claim 119, further comprising: creating relativemovement in a first direction between the plurality of beamlets and thetarget; and wherein the step of deflecting the electron beamletscomprises deflecting the beamlets in a direction different from thefirst direction.
 122. The method according to claim 119, wherein thestep of deflecting the electron beamlets comprises applying alternatingvoltages on consecutive strips of the electrostatic scan deflectorelectrodes.
 123. The method according to claim 119, wherein the step ofproviding an array of electrostatic scan deflectors comprises providinga first group of strips and a second group of strips, the first group ofstrips being arranged to scan in a first direction, the second group ofstrips being arranged to scan in a second direction.
 124. The methodaccording to claim 123, wherein the first direction is opposite to thesecond direction.
 125. A method of operating an electron optical systemcomprising an electron beam generator for generating an electron beam, atarget holder for holding a target on to which one or more beamlets areto be projected, and at least one of: a beam splitter for splitting theelectron beam into a plurality of beamlets; one or more electrostaticlens arrays for focusing the beam or beamlets; a modulation arraycomprising a plurality of modulators for modulating the beam orbeamlets; a deflector array comprising a plurality of electrostaticdeflectors for deflecting the beam or a portion of the beamlets in apredetermined direction; and a second electrostatic lens array forfocusing the deflected beamlets; the method comprising operating thesystem at an elevated temperature.
 126. The method according to claim125, wherein oxygen is admitted to the system.
 127. The method accordingto claim 125, wherein operating the system is performed at a temperatureabove 150 C.
 128. The method according claim 125, wherein operating thesystem is performed at a temperature below 400 C.
 129. The methodaccording to claim 125, wherein the operating temperature is elevatedsufficiently to effect a reduction of contamination of the system. 130.The method according to claim 129, further comprising preheating theelectron optical system at a temperature between 1000 and 1500 C.
 131. Amethod of operating an electron optical system comprising an electronbeam generator for generating an electron beam, a target holder forholding a target on to which one or more beamlets are to be projected,and at least one of: a beam splitter for splitting the electron beaminto a plurality of beamlets; one or more electrostatic lens arrays forfocusing the beam or beamlets; a modulation array comprising a pluralityof modulators for modulating the beam or beamlets; a deflector arraycomprising a plurality of electrostatic deflectors for deflecting thebeam or a portion of the beamlets in a predetermined direction; and asecond electrostatic lens array for focusing the deflected beamlets; themethod comprising the step of admitting oxygen to the system duringoperation.